The present invention relates to an ink jet head having a number of nozzles for discharging an ink drop, located with a high density. More specifically, the present invention relates to an ink jet head incorporated in an ink jet printing machine and configured to shoot an ink drop to a recording medium to produce a printed image recording, the ink jet head including a plurality of nozzles, a plurality of chambers located for the plurality of nozzles, respectively, a pressure plate forming one plane of each chamber, and an actuator provided for each pressure plate, so that when the actuator is driven, a corresponding pressure plate is deformed to compress an ink in a corresponding chamber so as to cause an ink drop to be discharged from a corresponding nozzle.
Now, the prior art of the ink jet head will be described with reference to FIG. 11, which illustrates one example of the prior art ink jet head disclosed in Japanese Patent Application Pre-examination Publication No. JP-04-148936-A. This prior art ink jet head includes a number of nozzles 101 arranged in a single array to depict a straight line, a number of chambers 102, each having an elongated plan view shape, staggered at opposite sides of the nozzle array, with one end of each chamber being located on a corresponding nozzle, and the other end of each chamber having a supplying port 103.
Furthermore, the ink jet head has an ink pool 104 in common to all the chambers, at a layer different from the layer where the chambers are formed. The ink pool 104 is connected to each chamber 102 through the associated supplying port 103. In addition, an actuator (not shown) is mounted on a pressure plate (not shown) forming one plane of each chamber. The following description will be made on the premise that the actuator is constituted of a piezoelectric actuator.
The ink jet head operates as follows: When the actuator is driven, the pressure plate is flexed to reduce the volume of the chamber 102, with the result that the ink in the chamber is compressed so that an ink drop is discharged through the nozzle 101. After the ink drop is discharged, when the pressure plate restores its original form, the ink is re-charged into the chamber through the supplying port from the ink pool 104
The above mentioned prior art ink jet head is advantageous since it is possible to realize a high density location of nozzles by reducing the pitch of the nozzles in the nozzle array and by locating a number of chambers having a narrow witch corresponding to the reduced pitch.
However, since the width of the chamber must be inevitably reduced with increase of the nozzle density, the pressure plate assumes the shape difficult to flex, with the result that it is no longer possible to ensure a necessary amount of volume change in the chamber. For reference, it is said that the amount of volume change of at least 30 pl (picoliter) is required to realize a high speed printing. If only a too small ink drop is discharged, the printing speed unavoidably becomes low.
In this connection, it may be considered to enlarge the longitudinal length of the chamber so as to increase the amount of volume change in the chamber. In this case, however, the occupying area of the chamber becomes large, with the result that the areal density of the nozzles cannot be elevated.
Accordingly, it is an object of the present invention to provide an ink jet head which has overcome the above mentioned problems of the prior art.
Another object of the present invention is to provide an ink jet head including a chamber having a pressure plate of the shape easy to flex, thereby to be able to reduce the occupying area of the chamber while maintaining a necessary volume of discharged ink drop, so as to realize a high density of nozzle location.
Still another object of the present invention is to provide a stably driven, highly reliable, ink jet head capable of maintaining a necessary volume of discharged ink drop even if the nozzles are located with a high density.
A further object of the present invention is to provide an ink jet head which can be manufactured with a simplified fabricating process and with a high working precision.
The above and other objects of the present invention are achieved in accordance with the present invention by an ink jet head comprising:
a plurality of nozzles arranged to constitute a matrix having a plurality of rows inclined at a constant angle to a head main scan direction and a plurality of columns orthogonal to the head main scan direction;
a plurality of chambers provided for the plurality of nozzles, respectively;
a pressure plate formed to constitute one plane of each of the plurality of chambers;
an actuator provided to each pressure plate, for deforming the pressure plate to compress an ink within a corresponding chamber;
a plurality of ink pool branches provided along the plurality of rows or the plurality of columns, for supplying the ink to the plurality of chambers; and
a main ink pool connected to the plurality of ink pool branches,
wherein the pressure plate has a plan view shape fulfilling the relation of 1xe2x89xa6Axe2x89xa62 where xe2x80x9cAxe2x80x9d is the ratio of an circumscribed circle diameter of the plan view shape of the pressure plate to an inscribed circle diameter of the plan view shape of the pressure plate.
For example, the pressure plate has the plan view shape of a substantial square, or alternatively the plan view shape of a substantial rhombus having a pair of sides in parallel to the plurality of rows and another pair of sides in parallel to the plurality of columns.
With this arrangement, the pressure plate has the shape easy to be flexed by the actuator when the actuator is driven. Therefore, even if the occupying area of the chamber is reduced, a highly efficient chamber can be realized which can maintain a necessary amount of volume change caused when the actuator is driven. Accordingly, a number of nozzles can be located with a high density without reducing the volume of the discharged ink drop.
When the pressure plate has the plan view shape of a substantial rhombus having a pair of sides in parallel to the plurality of rows and another pair of sides in parallel to the plurality of columns, the actuator preferably has a plan view shape of a substantial rhombus having a pair of sides in parallel to the plurality of rows and another pair of sides in parallel to the plurality of columns.
With this arrangement, the actuators can be formed in bundle by dicing an actuator material sheet in units of row and in units of column. Therefore, the fabricating process can be simplified, and it is possible to elevate the precision in the size and in the position of the actuator.
In addition, each of the plurality of chambers preferably has an ink supply port to form an ink flow in the direction of a long diagonal of the substantial rhombus.
With this arrangement, an ink flow within the chamber is smoothed, so that it is possible to prevent a bubble from staying, thereby to elevate reliability of the ink jet.
When the inclination of each row to the head main scan direction is expressed by xe2x80x9cxcex8xe2x80x9d and the interval of the plurality of nozzles in a direction orthogonal to the head main scan direction is expressed by xe2x80x9cdxe2x80x9d (mm), xe2x80x9cxcex8xe2x80x9d and xe2x80x9cdxe2x80x9d are preferred to have the relation of 0 less than tan xcex8xe2x89xa65d.
With this arrangement, the width of each chamber in the main scan direction can have the size of not less than about 0.2 mm, so that it is possible to obtain the chamber size capable of generating the amount of volume change sufficient to discharge a desired volume of ink drop.
Furthermore, when the number of the nozzles included in each row is expressed by xe2x80x9cnxe2x80x9d, xe2x80x9cnxe2x80x9d and xe2x80x9cdxe2x80x9d are preferred to have the relation of nxc3x97dxe2x89xa70.2.
Also with this arrangement, the width of each chamber in the direction orthogonal to the main scan direction can have the size of not less than about 0.2 mm, so that it is possible to obtain the chamber size capable of generating the amount of volume change sufficient to discharge a desired volume of ink drop.
Furthermore, xe2x80x9cnxe2x80x9d and xe2x80x9cxcex8xe2x80x9d preferably have the relation of 0.5xe2x89xa6nxc3x97tan xcex8xe2x89xa62.
With this arrangement, each chamber can have the head main scan direction width which is not greatly different from the width in the direction orthogonal to the head main scan direction. Therefore, according to this layout, the chambers can be located with a high density in the form of a matrix with no substantial wasteful space. For example, in the case of nxc3x97tan xcex8 less than 1, each chamber can have the head main scan direction width larger than the width in the direction orthogonal to the head main scan direction. On the other hand, in the case of nxc3x97tan xcex8 greater than 1, each chamber can have the head main scan direction width smaller than the width in the direction orthogonal to the head main scan direction. Accordingly, when each chamber assumes a plan view shape of the substantial square or the substantial rhombus, an empty space can be effectively used as a space for the ink pool branches, so that the chamber and the branches can be located with a high density.
Further, it is preferred that the main ink pool has the cross-sectional area larger than the cross-sectional area of the ink pool branch.
With this arrangement, even if all the ink flows flowing through all the ink pool branches are caused to flow through the main ink pool, a stably stream can be obtained. In addition, since a flow resistance is small, the supply amount of ink per a unit time can be enlarged, and therefore, a discharging frequency can be increased. Furthermore, a difference in the ink supplying amount between the ink pool branches can be reduced so that the variation in the discharge characteristics between the nozzles can be suppressed.
Moreover, it is also preferred that the main ink pool and/or each ink pool branch has the cross-sectional area gradually decreasing toward the downstream end.
In the main ink pool, the ink flow rate drops to toward the downstream end, since the ink is supplied to the associated ink pool branches. In addition, in each ink pool branch, the ink flow rate also drops to toward the downstream end, since the ink is supplied to the associated chambers. With this arrangement, however, it is possible to prevent the flow velocity from dropping. Accordingly, the ink can be stably supplied to the chambers positioned at the downstream side of the ink flow, so that the variation in the discharge characteristics between the nozzles can be suppressed. Furthermore, it is possible to prevent stay of a bubble, which would have been caused because of the drop of the flow velocity, thereby to elevate reliability of the ink jet.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.